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I suspect Sierra Pacific [0].

[0] https://en.wikipedia.org/wiki/Sierra_Pacific_Industries

Weyerhaeuser owns something in the neighborhood of 12 million acres of timberland in the US, though I don't know what their largest contiguous parcel is. They are probably the largest private landowner in the US (corporate, not individual).

http://fortune.com/2012/09/27/meet-the-largest-landowner-in-...

Maybe the King Ranch in Texas?

https://en.wikipedia.org/wiki/King_Ranch

For an indicator of scale -- the King Ranch is four separate plots that aren't contiguous covering 825,000 acres.

Anna Creek Station covers 5,850,000 acres. https://en.wikipedia.org/wiki/Anna_Creek_Station